1. Field of the Invention
The present invention relates to power-line carrier communications systems, and more particularly to a system for digital communications using a power-line in which transmission-path characteristics and noise fluctuate in synchronization with power supply frequency.
2. Description of the Background Art
A power-line is not designed for data communications, but generally is only for transferring power. Further, impedance matching is not done for branched parts and terminations thereof. Consequently, using the power-line as a transmission path for data communications leads to harsh environments (frequency-gain characteristics or noise, for example).
Although the frequency-gain characteristics of the power-line show a tendency that a signal of lower frequency is attenuated comparatively less, attenuation characteristics differ according to transmission paths. Accordingly, signals at certain frequencies may not be transmitted due to resonance, and thus the characteristics cannot be uniquely specified.
Further, although a noise level in the power-line is comparatively high at lower frequencies, narrow-band noise, for example, may be involved, whereby the noise level also cannot be uniquely specified. Accordingly, such dependence on the transmission path renders a signal-to-noise ratio (S/N) at a reception point hard to specify at which frequency the ratio being the largest.
Likewise, when the power-line is distorted, no one can define where resonance will occur. Consequently, selecting a less-distorted frequency band for signal transmission is difficult.
Still further, the aforementioned characteristics are not static. In other words, devices connected to the power-line ceaselessly change in operation. Therefore, the devices' change in operation involves load characteristics' change, thereby dynamically changing the transmission-path characteristics and noise level, for example.
FIG. 8 exemplarily shows the gain-frequency characteristics of the power-line. As shown in FIG. 8, the gain characteristics of the power-line differ according to frequencies. FIG. 9 exemplarily shows the noise-frequency characteristics of a device. As shown in FIG. 9, a harmonic noise such as an inverter noise is concerned in the noise characteristics of device. Note that FIGS. 8 and 9 are only examples, and thus frequency and noise characteristics vary according to transmission paths or devices.
In such a general narrow-band communications system, it is difficult to appropriately cope with attenuation in the frequency bands or generation of noise shown in FIGS. 8 and 9. This is because no one can tell which signal at which frequency becomes non-transmissible due to attenuation caused by resonance or which signal at which frequency is affected by the harmonic noise.
Therefore, various techniques have been proposed so far to solve such problems by providing redundancy through a spread spectrum modulation scheme. Especially, a communications system in which a direct sequence scheme of spread spectrum is applied is noteworthy. Hereinafter, a conventional power-line carrier communications system in which the direct sequence spread spectrum system is described.
FIG. 10 is a block diagram exemplarily showing the conventional power-line carrier communications system in which the direct sequence spread spectrum system is applied. In FIG. 10, the conventional power-line carrier communications system is structured by a transmission apparatus 300 and a reception apparatus 400 interconnected through a transmission path 500. The transmission apparatus 300 includes a modulation part 301, a spread spectrum modulation part 302, a spread code part 303, and a transmission amplifier 304, while the reception apparatus 400 includes a reception amplifier 401, a spread spectrum demodulation part 402, a spread code part 403, and a demodulation part 404.
First, the transmission apparatus 300 is structurally described. A transmitting symbol is inputted into the modulation part 301. By using the received transmitting symbol, the modulation part 301 modulates a carrier in a predetermined arbitrary manner, and then outputs the modulated carrier. The spread code part 303 carries a spread code whose chip rate is higher than the symbol rate from the modulation part 301, and then outputs the spread code to the spread spectrum modulation part 302. The spread spectrum modulation part 302 multiplies the modulated carrier by the spread code so as to spread a spectrum on a frequency axis (widen a frequency band). The spread-spectrum-modulated transmitting signal is amplified in the transmitting amplifier 304 so as to be in a predetermined amplitude, and then is transmitted to the reception apparatus 400 side through the transmission path 500.
Next, the reception apparatus 400 is structurally described. A receiving signal coming through the transmission path 500 is amplified in the reception amplifier 401 so as to be in a predetermined amplitude, and then is outputted to the spread spectrum demodulation part 402. The spread code part 403 carries the same spread code as the spread code part 303 of the transmission apparatus 300 does, and outputs the spread code to the spread spectrum demodulation part 402. The spread spectrum demodulation part 402 subjects the receiving signal to inverse-spreading by multiplying the same by the spread code so that the widened frequency band is narrowed down to the original width. The inversely-spread receiving signal is demodulated in the demodulation part 404 (which corresponds to the modulation scheme applied in the modulation part 301 of the transmission apparatus 300), and then is outputted as a receiving symbol.
Although not being described in detail herein, such spread spectrum communications scheme has various features to synchronize the spread codes or to delay detection which enhances a distortion-resistance by referring to a preceding signal for inverse-spreading and simplifies a synchronization circuit.
As is known from the above, the conventional power-line carrier communications system spreads information on a frequency axis through spread spectrum. To be more specific, the conventional power-line carrier communications system provides a modulated signal with redundancy on a frequency axis and then transmits the resultant signal. Therefore, even if a signal is attenuated at a certain frequency or has a high noise level, the signal can be demodulated with help of other frequency components.
In such conventional power-line carrier communications system, however, the signal is redundant only on the frequency axis through spread spectrum. Accordingly, the system still has problems as described hereafter.
In the power-line, noise to be generated over time in devices connected thereto is more concerned than noise to be generated in the frequency bands. FIGS. 11A and 11B exemplarily show how the noise temporally changes. In both FIGS. 11A and 11B, an upper waveform is a noise waveform, and a lower is a trigger signal synchronizing with a power cycle. For the trigger signal, presumably, two clocks are equal to a power-supply cycle (herein, power-supply frequency is assumed to be 60 Hz).
As is obvious from FIGS. 11A and 11B, the noise level shows 120 Hz-periodicity in its fluctuation, which is twice as much as the power-supply cycle. Thus, noise in the power-line is typically synchronizing with the frequency being twice as much as the power-supply cycle.
In this manner, in the power-line carrier, S/N differs according to frequency bands on the frequency axis as shown in FIGS. 8 and 9. Further, as shown in FIGS. 11A and 11B, the noise level temporally varies on the time axis. Especially, impulse noise in FIG. 11B is wide in frequency and high in level, and whereby redundancy provided only on the frequency axis is not adequate to allow proper communication at the time when the high-level noise is observed.
Specifically, as is done in the conventional power-line carrier communications system, redundancy provided only on the frequency axis is not sufficient to cope with noise on the time axis, and accordingly errors occur during communications.